Method and devices for controlling a vapour flow in vacuum evaporation

ABSTRACT

A method and a device for the coating of running substrates moving along a run direction through a treatment zone, in which the vapor of a coating material is generated in a chamber, this vapor passing through g a treatment aperture towards the treatment zone where the coating material condenses on the surface of the substrates. The vapor flow through the treatment aperture is controlled by adjusting the extent to which the treatment aperture is shut off by at least one shutter, between an open position, in which the vapor flows through the treatment aperture towards the treatment zone, and a closed position, in which the vapor is prevented from flowing towards the treatment zone through the treatment aperture.

1. GENERAL DESCRIPTION OF THE INVENTION

The invention relates to a method and device for the coating of runningsubstrates, moving in a direction of travel, this device comprising aconfinement enclosure in which a source of vapour of the coatingmaterial is present and having a treatment zone communicating with thesource of vapour through at least one treatment opening. The source ofvapour makes it possible to generate a flow of vapour for coating asubstrate. The invention therefore concerns a device and method forcontrolling a flow of vapour towards a substrate to be coated in avacuum evaporation coating method. The invention is particularlyadvantageous for the control and regulation of a flow of zinc vapour,obtained by evaporation of the metal, towards steel substrates in theform of strips, girders, plates, profiles, with all types of transversesections, but also parts disposed on supports, for example hooks ormetal baskets, transported in the coating zone.

Although the control of the flow of evaporated vapour that is thesubject matter of the invention is completely independent both of thenature of the substance evaporated and the type of evaporation used, itis particularly well suited to the control and regulation of a flow ofzinc vapour towards the substrate to be coated in a method ofgalvanisation by zinc plasma evaporation. The coating material containedin the vapour source may for example be heated by Joule effect orinduction or by plasma in order to make this material evaporate.

The galvanisation method by zinc plasma evaporation is already known anddescribed in the document WO 02/16664. This method uses a retentionvessel for maintaining a certain quantity of zinc in the liquid stateand evaporating by means of a plasma produced in the zinc vapour by onaverage negatively biasing the liquid zinc with respect to acounter-electrode, in particular an anode. The counter-electrode may beformed by the substrate to be coated. The retention vessel is suppliedwith liquid zinc via a supply tube immersed in a reserve of zincmaintained in a vacuum furnace situated in a vacuum enclosure, isolatedfrom any passage of gas towards the galvanisation vacuum tank, and inwhich, by regulating the gas pressure, it is possible to regulate thelevel of liquid zinc in the retention vessel situated in the vacuum tankwhere the galvanisation takes place. The plasma produced in the zincvapour is generally obtained by means of a magnetron discharge by meansof a magnetic circuit disposed under the retention vessel. The zincvapour tension above the retention vessel depends on the electricalpower dissipated at the surface of the liquid zinc and fixes the weightof zinc that it is possible to deposit per unit of time on the steelsubstrate. This vapour tension may be as much as several mbarcorresponding generally to a mass of zinc evaporated of several kg/min.It is therefore recommended, as already described in the document WO02/16664, to provide a confinement enclosure with heated walls in orderto prevent the zinc vapour contaminating the whole of the installationby condensing over all the cold surfaces, which are generally at ambienttemperature, other than those of the substrate. Entry and exit openingsare provided in this confinement enclosure in order to enable thesubstrate to be coated to pass through it. The zinc coating is thereforeobtained by condensation of the zinc vapour, directly in the solid stateon the cold surface of the substrate passing through the confinementenclosure. The temperature of the surface of the substrate is typicallybelow 150° C.

It will easily be understood that it is necessary to be able to adaptthe electrical power delivered to the plasma and dissipated, viabombardment by ions issuing from the plasma, on the surface of theliquid zinc in the retention vessel according to the area of the steelsubstrate passing per unit of time through the confinement enclosure. Inparticular, cutting this electrical supply where no substrate ispresent, or is not passing, in the confinement enclosure, and converselytriggering and progressively increasing the electrical power dissipatedon the surface of the liquid zinc when the substrate enters, or beginsto pass in, the confinement enclosure. This is done so as not only toensure a uniform thickness of zinc on the surface of the substrate butin particular to limit the losses of zinc through the entry and exitopenings of the confinement enclosure when no substrate is passingthrough the confinement enclosure. This is because, apart from theeconomic loss, in terms of zinc and energy, the zinc contamination mayseriously damage the galvanisation installation by plasma evaporation ifit is not controlled.

The use of a confinement enclosure as described in the prior art hasseveral disadvantages.

This is because, when no substrate is present or passing through theconfinement enclosure, even if the electrical supply enabling zinc to beevaporated by plasma heating is cut off, for example, the liquid zinccontained in the retention vessel continues to evaporate because of theheat stored in the metal during normal functioning of the process. Thiszinc vapour is therefore liable to escape freely through the entry andexit openings of the confinement enclosure or to condense on an immobilesubstrate. To limit this loss of zinc towards the outside of theconfinement enclosure, or the condensation of the zinc on an immobilesubstrate, a simple means could consist of emptying the retention vesselof any presence of liquid zinc and returning it to the liquid zincreserve situated in the temperature-maintenance furnace in the tanksituated under the galvanisation tank. Unfortunately, the transient timenecessary for the entry or exit of a substrate to or from theconfinement enclosure, or the stoppage or restarting of the passage ofthe substrate, is generally much less than the time necessary forfilling and draining the retention vessel with liquid zinc through itsfeed tube. Consequently this solution cannot be envisaged in practice.

Moreover, when any substrate is passing through the confinementenclosure, the confinement enclosure, as described in the prior art,does not make it possible to adapt the flow of zinc according to thevarious faces of the substrate to be coated. This could be important forthe purpose of ensuring uniform deposition of the entire substratewhereas the area of the surface of the substrate to be coated varieswith the orientation of the vapour flow, as is for example the case witha girder with an I-shaped or U-shaped cross section. It is also notpossible to coat the substrate with different thicknesses on itsdifferent faces, for example, when it is wished to apply a coating witha certain thickness to one of the faces of a steel sheet that isdifferent from the thickness of the coating applied to the other face ofthis sheet.

The method and devices of the subject matter of the invention aimessentially to solve among other things, these two drawbacks by makingit possible:

to isolate the inside of the confinement enclosure from the treatmentzone where the substrate to be coated is passing. This function isactivated when no substrate is traversing the treatment zone ortravelling through this treatment zone;

to regulate the vapour flow according to its orientation towards thesubstrate transversely with respect to the direction of travel, oraccording to the position of the vapour source in the confinementenclosure with respect to the treatment zone, by regulating at least onevapour obturation element.

To this effect, regulation means are provided for controlling the flowof said vapour between the vapour source and the treatment zone throughsaid treatment opening. These regulation means comprise obturationelements able to be moved between an open position in which the flow ofsaid vapour can pass through the treatment opening towards the treatmentzone, and a closed position in which the treatment zone is isolated withrespect to the vapour source.

In a particularly advantageous configuration of the invention, a passageextends in the direction of travel of the substrates between an entryopening and an exit opening of the confinement enclosure, said treatmentzone being situated in this passage and the regulation means being suchas to make it possible to control the flow of said vapour between saidvapour source and the inside of the passage.

The entry and exit openings of the confinement enclosure are thusconnected together by a passage in the form of a tube passing completelythrough the confinement enclosure and having opening and closing devicesenabling vapour to pass from the inside of the confinement enclosuretowards the treatment zone through which the substrate is passing. Whenno substrate is traversing or travelling through the treatment zone, allthe openings of this tube passing right through the confinementenclosure are closed off. When a substrate is passing through thetreatment zone, the openings in the passage are more or less openaccording to the flow of vapour desired in line with each of themtowards the substrate.

A device for heating, for example by Joule effect, the wall of theconfinement enclosure is provided and sized so as to achieve a walltemperature sufficient to prevent any solid or liquid condensation ofthe metal vapour on said walls. This system for heating the confinementenclosure can also advantageously enable, by radiation, the maintenanceat the same temperature, preventing any condensation of the zinc vapour,of the walls making up the passage for regulating the flow of vapourpassing through the confinement enclosure. This makes it possible tosimplify the design of the vapour flow regulation device, which cansimply make use of a simple mechanical construction consisting ofobturation elements, generally of any geometry, able to be moved betweenan open position, in which the flow of said vapour can pass through thetreatment opening towards the treatment zone, and a closed position inwhich the treatment zone is isolated with respect to the vapour sourcegenerally with any geometry.

It is clear that the invention is not limited to a plasma galvanisationdevice and to a confinement enclosure through which a passage in theform of a tube with adjustable obturation elements passes right through.This is because vacuum evaporation could be obtained by any means ofheating by means of generally any vapour source. The material is notlimited to zinc. Other metals such as magnesium, for example, or organicmolecules with a view to depositing polymers are possible. The system ofadjustable obturation elements is not necessarily formed by a tubularstructure passing right through the confinement enclosure, but couldalso have a flat geometry closing off a treatment opening formed in awall of the confinement enclosure and facing an area of the substrate tobe coated.

2. DESCRIPTION OF THE FIGURES

Other details and particularities of the invention will emerge from thedescription given below by way of non-limitative example of a particularembodiment of the device and method according to the invention withreference to the accompanying drawings.

FIG. 1 shows, in perspective, a plasma galvanisation device, asdescribed in the prior art.

FIG. 2 is a schematic representation in perspective of a confinementenclosure according to the invention, the treatment zone of which, whichis provided with obturation elements formed by articulated slats,extends between an entry opening and an exit opening of the enclosure.

FIG. 3 is a schematic representation in perspective of the walls of thetreatment zone, which are formed by a succession of articulated parallelslats, according to the invention.

FIG. 4 is a schematic representation in perspective of a confinementenclosure with a treatment zone the walls of which are formed byobturation elements in the form of slides, according to the invention.

FIG. 5 is a schematic representation in perspective of a confinementenclosure with a treatment zone the walls of which form flat obturationelements that can be moved in parallel or perpendicularly with respectto their plane, according to the invention.

FIG. 6 is a schematic representation of a particularly advantageous formof the device according to the invention in a plasma galvanisationinstallation, when substrates pass through the treatment zone and theobturation elements are in their open position.

FIG. 7 shows a device according to the invention of FIG. 6 in theabsence of substrates and when the obturation elements are in theirclosed position.

In the various figures, the same reference signs refer to analogous oridentical elements.

3. DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the galvanisation device according to the prior artconsists of top equipment 1 through which the substrate to be coatedpasses and bottom equipment 2 comprising a liquid zinc reserve.

The top equipment 1 comprises a vacuum chamber 3 in which there isarranged a vapour source formed by a retention vessel 4 surmounted by aconfinement enclosure 5. The confinement enclosure has heated walls inorder to prevent the deposition of vapour on these walls and is tubularin shape enabling steam to circulate freely and be distributed allaround the product to be coated without any leakage of vapour in adirection transverse to the direction of movement of the substrate to becoated.

The liquid zinc that is present in the retention vessel 4 is evaporatedby a plasma generated in the confinement enclosure 5 in order to createa flow of zinc vapour towards a treatment zone 6 that the substrate tobe coated passes through. This treatment zone 6 extends between an entryopening 7 and an exit opening 8 of the confinement enclosure 5 andcorresponds to a passage that connects the openings 7 and 8 of theconfinement enclosure 5. The treatment zone 6 is situated above theretention vessel 4 and communicates with the inside of the confinementenclosure 5 through the open walls of this passage, which thus formtreatment openings.

The retention vessel 4 is supplied by means of a supply tube 9 passingthrough a gastight connection 10 situated between the vacuum tank 3 ofthe top equipment 1 and a second vacuum tank 11 of the bottom equipment2. This supply tube 9 emerges in the bottom of the retention vessel 4and is immersed in a liquid zinc reserve stored in atemperature-maintenance furnace 12 in the second vacuum tank 11.

As is clear in this FIG. 1, the confinement enclosure 5 according to theprior art does not comprise any device for regulating the flow of zincvapour towards the various faces of the substrate that will be presentin the treatment zone 6 or for blocking the passage of this vapour whereno substrate is present in the treatment zone 6.

FIG. 2 represents the confinement enclosure 5 with the treatment zone 6of FIG. 1 having regulation means for controlling the flow of vapourfrom the coating material between the vapour source 4 and the treatmentzone 6 through the treatment openings 14, according to an advantageousembodiment of the invention.

The space inside the confinement enclosure 5 makes it possible todistribute the vapour from the coating material all around the productto be coated and/or around the passage. Thus a free space is preferablypresent between the walls of the confinement enclosure and the walls ofthe passage allowing free circulation and a distribution of the vapourin the confinement enclosure.

The passage, which comprises the treatment zone 6, preferably has atubular structure and is delimited by walls that extend between theentry opening 7 and the exit opening 8. Thus the walls of the passagesurround the treatment zone 6. These walls have the treatment openings14 and comprise the regulation means for closing or opening thesetreatment openings 14 and regulating the flow of vapour in directionstransverse to the direction of movement of the substrate.

The regulation means comprise obturation elements 13 that can be movedbetween an open position which the flow of said vapour can pass throughthe treatment opening towards the treatment zone 6, and a closedposition in which the treatment zone 6 is isolated with respect to thevapour source 4.

The walls of the passage, which extends in the direction of travel ofthe substrates between the entry opening 7 and the exit opening 8 of theconfinement enclosure 5 and which comprises said treatment zone 6, areformed by obturation elements 13 that can be moved between an openposition and a closed position. In the open position, the treatmentopening is at least partially open and the flow of said vapour can enterinside the passage from the vapour source 4 through this treatmentopening. In the closed position the inside of the passage, that is tosay the treatment zone, is isolated by the obturation elements 13 withrespect to the vapour source 4. Thus the walls of the passage can bemoved between a closed position and an open position.

The obturation elements 13 are formed by slats 15 that are articulatedwith respect to the treatment opening so as to enable them to tiltbetween the open position and the closed position. The slats 15 have anoblong shape and their longitudinal direction extends parallel to thedirection of movement of the substrates between the entry opening 7 andthe exit opening 8 of the confinement enclosure 5.

In FIG. 3 the walls of the passage are shown. These walls comprise saidobturation elements 13 formed by the slats 15 and thus constitute a kindof venetian blind that can close off the treatment opening in order toisolate the treatment zone with respect to the confinement enclosure 5.

The passage has a tubular structure composed of walls having rectangularslats 15 parallel to one another and fixed by a middle movable spindlein support frames 16. The support frames 16 have openings 17 that areadjacent to respectively the entry opening 7 and the exit opening 8 ofthe confinement enclosure 5 and enable the substrate to be coated topass. By rotation about the median axis of the slats 15, the latter canbe opened or closed like blinds. A device for causing the rotation ofthe slats is not shown for reasons of clarity of the figure. Therotation of each slat 15 can take place in a generally independentmanner. This gives the greatest flexibility to the coating method. Thedevice for driving the slats 15 can comprise for example motors orjacks, and is advantageously situated outside the confinement enclosure5, in order not to be contaminated by the vapour produced by the vapoursource 4.

FIG. 4 shows a confinement enclosure 5 with a treatment zone 6 that isdifferent from the one shown in FIG. 2 through the fact that the wallsof the passage containing the treatment zone are formed by slides 18,19, 20 and 21.

In this particular configuration of the device according to theinvention, the slides 18, 19, 20 and 21 can be moved parallel to thecentral axis of the passage or, in other words, parallel to thedirection of travel of the substrates through the treatment zone 6. Bypulling these slides 18, 19, 20 and 21 outside the confinement enclosure5, the opening enabling vapour to pass towards the treatment zone in adirection perpendicular to the pulled slide is opened proportionally tothe pulled area of the slide 18, 19, 20 or 21 concerned outside theconfinement enclosure 5. The slides 18, 19, 20 and 21 are brought insidethe enclosure 5 in order to close off the entire treatment opening andto prevent the passage of vapour towards the treatment zone 6 when nosubstrate is passing through this treatment zone 6, and thus to minimisethe vapour losses towards the outside.

It is also possible for the walls or slides of the passage to consist ofone or more slats extending alongside each other to allow a more preciseregulation of the vapour flow around the substrate. In such a case, eachslat can be moved individually parallel to the direction of travel ofthe substrates.

The slides 18, 19, 20 and 21 are formed by plates that are for exampleactuated by means of jacks, not shown, and fixed to the external wallsof the enclosure 5.

FIG. 5 shows a treatment zone 6 that is provided in a passage, the wallsof which form flat obturation elements 13 that can be moved in parallelor perpendicularly with respect to the plane thereof.

This particular configuration of the device according to the inventionillustrates the fact that the obturation elements can be moved generallyin any manner. For example, perpendicular to their plane inside theconfinement enclosure 5 as is the case with the top obturation elementand the two lateral obturation elements. The bottom wall comprises forexample two obturation elements 13 that can be moved in their own planeand perpendicular to the direction of movement of the substrates, notshown. The substrates can travel through the openings 17 present in thesupport frame 16 on each side of the treatment zone 6.

FIGS. 6 and 7 show the device according to the invention, in a plasmagalvanisation installation. This installation comprises in particular asystem for transporting the substrates by banks of motorised rollers 24mounted in a vacuum tank 22 and 23 downstream and upstream of the deviceaccording to the invention, comprising the retention vessel 4 surmountedby the confinement enclosure 5 having the treatment zone 6. Naturallyother transport systems can be envisaged such as a monorail transportsystem for suspending the substrates and transporting them through thevarious zones of the installation. Means (26) are provided for creatinga plasma in the confinement enclosure (5) so as to evaporate the coatingmaterial and to polarise the latter on average negatively with respectto a counter-electrode.

The substrates are introduced into the installation by means of a vacuumairlock, not shown, which makes it possible to continuously maintain thetreatment zone 6 and the confinement enclosure 5 under vacuum or underthe required argon pressure and thus avoids contaminating the treatmentzone through the introduction of air. The required argon pressure istypically between 0.05 and 5 Pa.

The substrates leave the installation by means of an exit vacuum airlockthat is also not shown in the figures.

The device according to the invention shown in FIGS. 6 and 7 uses asystem for regulating the vapour flow having obturation elements 13 inthe form of slats 15. This system for regulating the flow of vapour ofthe coating material is shown in FIGS. 2 and 3. When the substrates 25to be galvanised pass through the treatment zone 6 of the top equipment1, the slats 15 are open in order to enable the zinc vapour to passtowards the substrates 25, as shown in FIG. 6.

In the absence of a substrate 25 passing through the treatment zone 6,the slats 15 for regulating the flow of zinc vapour towards thetreatment zone 6 are totally closed in order to limit to the maximumextent the zinc vapour losses in the vacuum chamber 3, as shown in FIG.7. Thus the walls of the passage are in their closed position so thatthe treatment zone 6 is isolated with a respect to the inside of theconfinement enclosure 5 where the vapour of the coating material ispresent.

4. OPERATIONAL CONDITIONS AND PARTICULAR CONFIGURATIONS OF THE INVENTIONIN A PLASMA GALVANISATION INSTALLATION

4.1. Starting the Plasma Evaporation Zinc Plating Unit

When the plasma evaporation zinc plating installation is put undervacuum after maintenance, the obturation elements formed by the flaps 13of the baffles of the vapour regulation device are arranged in positionssuch that the adjustable openings of the device are open to the maximumto enable the evacuation of the air and then the required pressurisationof argon inside the confinement enclosure. When the installation reachesan argon pressure of between 0.001 and 0.01 mbar the walls of theconfinement enclosure are heated by electric elements to a temperatureof between 400° and 500° C. The flaps 13 of the baffles of the zincvapour flow regulation device according to FIGS. 2 and 3 are actuated byrotation so as to close the openings and thus optically isolate thetreatment zone 6 from the inside of the confinement enclosure. The flaps13 of the baffles of the vapour regulation device are heated by theinfrared radiation emitted by the internal walls of the confinementenclosure 5. Advantageously, a few flaps 13 are provided withthermocouples for monitoring the temperature thereof. When the workingtemperature is reached on the flaps 13 (typically between 400° and 500°C.) the liquid zinc is introduced to its set level measured by generallyany electrical, optical or mechanical means, in the retention vessel 4.

4.2. Particular Device for Driving the Obturation Elements Consisting ofSuccessive Slats that can be Swung Between an Open Position and a ClosedPosition for Regulating the Zinc Vapour Flow Transversely to theDirection of Movement of the Substrate

In a particularly advantageous form of the invention, the obturationelements 13 formed by slats 15, as shown in FIGS. 2 and 3, are supportedon spindles fixed to the support 16. The rotation of a group of slats 15is made integral by generally any means, such as for example a chain,cables or a lever cooperating with a runner. This is done for the fourwalls of the passage that extend parallel to the direction of travel ofthe substrates. In this way the device can be opened in a regulatedfashion. In particular, the treatment opening for each of the walls ofthe passage containing the treatment zone can be opened and closedindependently. These walls are respectively situated above, below andlaterally with respect to the treatment zone 6 of the substrate andextend between the entry opening 7 and the exit opening 8.Advantageously, the mechanical driving of the obturation elements 13 isdone by means of four independent devices fixed to the external walls ofthe confinement enclosure 5 maintained at ambient temperature(approximately 300K) via four transmissions passing through the walls ofthe confinement enclosure 5. The mechanical drive devices may be jacksor rotary devices and may generally be electromechanical, pneumatic orhydraulic.

The advantage of this particular configuration of the device accordingto the invention is to be able to regulate zinc vapour flow by directionaccording to the area of the substrate to be coated dependent on thetransverse direction in question for a substrate of given geometry. Itis in particular possible to regulate the distribution of the passage ofthe vapour towards the treatment zone according to the transversesection of the substrates to be coated.

Thus, for example, for a steel substrate with a U-shaped transversesection, the opening of which is oriented upwards and the principaldimensions in cross section of which are equivalent, ignoring thethickness of the substrate, it is necessary to have a zinc vapour flowthat is three times greater through the wall of the passage situatedabove the substrate than through the walls situated below and on thesides of the substrate since the internal surface of the U is threetimes greater than each of the lateral and bottom surfaces. The internalsurface of the U is essentially coated with zinc passing through the topwall. This top wall will therefore be totally open whereas the lateralwalls and the bottom wall will be partially closed.

In general, the degree of closure of each wall depends not only on thegeometry of the substrate but also on the size of the substrate withrespect to the transverse dimensions of the cross section of the passageconstituting the treatment zone, the number and distribution of thesubstrates in the treatment zone when several substrates are treated inparallel, and the proximity of the vapour source with respect to eachwall.

By way of example, if three substrates, with a U-shaped transversesection as described above, are treated in parallel and distributed atequal distances in the treatment zone, the characteristic openings ofthe walls that might lead to a uniform coating over all the faces of thesubstrate could typically be: 100% opening for the top surface, 60% forthe bottom surface and 20% for the two lateral surfaces.

In the case of the zinc plating of a steel sheet or plates, for thepurpose of obtaining a thickness of deposit that is uniform on the twofaces of a sheet or plate moving horizontally in the treatment zone 6above the retention vessel 4, it will be necessary to completely openthe obturation elements 13 situated above the sheet or plate andpartially close the obturation element situated between the retentionvessel and the sheet or plate so as to regulate the flows of zinc vapourin the confinement enclosure and to obtain an identical zinc vapour flowon the two faces of the sheet or plate. In general, the obturationelement situated opposite the lateral edges of the sheet or plate arenot necessary for the application of the device according to theinvention to the zinc plating of a steel sheet or plates. Use canadvantageously be made of fixed left and right lateral walls closing offany passage of vapour in a direction perpendicular to these walls.

4.3. Management of the Plasma Evaporation Zinc Plating Unit Providedwith a Device According to the Invention in Operating Mode

When no substrate is situated in the treatment zone 6 inside thepassage, the obturation elements 13 are held in positions such that allthe walls or treatment openings are closed.

When the substrate is detected as being situated partially or totallyinside the treatment zone by one or other or both detectors situated inthe vicinity of the entry and exit openings 7 and 8, the obturationelements 13 are actuated to enable the walls to open, thus providingpassage of the zinc vapour towards the substrate to a required degreeaccording to the required zinc vapour flow and/or according to theorientation of the obturation elements with respect to the direction oftravel of the substrate. The degree of opening of the obturationelements is generally pre-regulated according to the characteristics ofthe substrates to be treated, for example according to the geometry andnumber of substrates treated in parallel.

When the obturation elements 13 open, the electrical supply (means forcreating plasma 25) connected to the retention vessel 4 and to an anode,not shown and situated in the confinement enclosure 5 between theretention vessel 4 and the passage comprising the treatment zone, andenabling plasma to be obtained in the zinc vapour necessary for theevaporation of the liquid zinc introduced into this retention tank, istriggered. The power delivered to the plasma by this electrical supplyis increased according to a pre-established program up to its nominalpower when the two entry and exit detectors are activated by thesubstrate. Finally, when only the detector situated in the vicinity ofthe exit opening 8 of the enclosure 5 is activated by the substrate, thepower delivered by this electrical supply is reduced until it is cut offaccording to a pre-established program. When the substrate hascompletely left the treatment zone and is no longer detected by thedetector situated in the vicinity of the entry and exit openings 7 and 8of the confinement enclosure 5, the obturation elements 13 are actuatedin order to completely close off the treatment openings in the walls ofthe passage and thus minimise the zinc vapour losses outside theconfinement enclosure 5.

An important advantage of the method according to the invention,afforded by the closure of the treatment zone with respect to theconfinement enclosure containing the zinc vapour, when no substrate ispassing through it or travelling therein, is that, preventing the lossesof zinc by evaporation to the treatment zone of the substrate, thesystem is maintained at thermodynamic equilibrium, at the temperature ofthe walls of the confinement enclosure, without having to maintain theelectrical power in the plasma, while preventing any risk ofsolidification of the liquid zinc in the retention vessel.

4.4. Stoppage of the Plasma Evaporation Zinc Plating Unit Provided withthe Device for Regulating the Vapour Flow

When the plasma evaporation galvanisation unit is stopped, no substrateany longer being detected in the treatment zone, the obturation elementsare in the closed position. The retention vessel 4 is then emptied toits temperature-maintenance furnace 12 via the supply tube 9. When thevessel 4 is emptied of all liquid zinc, the obturation elements arecompletely open in order to enable gases to pass between the inside andoutside of the confinement enclosure 5. The heating elements of theconfinement enclosure are then cut off in order to enable the internalwalls of the enclosure to cool before the installation is opened toatmosphere for maintenance.

5. EXAMPLES OF PRACTICAL APPLICATIONS

5.1. Installation for the Plasma Evaporation Galvanisation of SteelProfiles

An installation for galvanising steel profiles by plasma evaporation isequipped with confinement chambers provided with a device for regulatingthe zinc vapour flow with articulated slats 15, as shown in FIGS. 2 and3. The vapour flow regulator is provided with four independent wallsaffording independent regulation of the zinc vapour flow respectivelyabove, below and on the sides of the charge passing through thetreatment zone. The obturation elements of the four walls correspondingto the four principal transverse orientations in the direction of travelof the charge to be galvanised are actuated by rotation of thecorresponding slats 15 by means of jacks (not shown) fixed to theexternal walls of the confinement enclosure 5. The cross section of theopening 17 for the passage of the substrates at the support frames 16measures typically 700 mm×200 mm and the dimension of the passage or ofthe treatment zone parallel to the direction of travel of the substratemeasures 600 mm. When the substrates are detected as passing through thetreatment zones, the slats 15 are opened for each of the fourindependent walls according to a degree of opening allowing uniformcoating of the substrates on all their faces. Typically, the wallsituated above the substrates is 100% open, the wall situated below thesubstrates is 80% open, and the lateral walls are 50% open. When thecharge has completely left the treatment zone, the independentobturation elements are completely closed and the electrical supplyconnected between the retention vessel and the anode facing it (notshown) is cut so as to minimise as far as possible the zinc lossestowards the outside of the confinement enclosure. This device forregulating the zinc vapour flow affords transverse regulation around thecharge of a nominal flow of zinc vapour of around 3.5 kg per minute.

5.2. Installation for the Plasma Evaporation Galvanisation of RunningSteel Sheets

An installation for the galvanisation of running steel sheets by plasmaevaporation is equipped with confinement chambers through which a 1 mmthick and 1 m wide steel sheet passes horizontally, at a speed of the100 m/min, one side of the sheet facing the retention vessel 4. Eachconfinement enclosure has a zinc vapour flow regulation device providedwith a top slide baffle situated above the sheet and a bottom slidebaffle situated below the sheet, as shown in FIG. 4, and actuatedindependently by fixed jacks on the external walls of the confinementenclosure (not shown). The lateral walls of the zinc vapour regulationdevice are fixed given the flat geometry of the sheet. The internaldimensions of the vapour flow regulation device are 1100×50 mm for theopening 17 in the support frames 16 and 600 mm for the longitudinaldimension in the direction of movement of the sheet. In operation, inorder to have a deposit of uniform thickness over the entire surface ofthe sheet, the top slide is 100% open whereas the bottom slide is openonly by 70% of its total opening.

This device is particularly advantageous for the galvanisation of steelsheets since it also makes it possible to completely dispense with theneed to use accumulators generally necessary in continuous coatingprocesses during changes of steel reel, which constitutes a verysignificant saving in investment. This is because, when the travel ofthe belt stops, in addition to the cutting of the electrical supplyenabling plasma zinc evaporation, the complete closure of the top andbottom slides of the vapour flow regulation device completely protectsthe sheet from any deposition of zinc during the belt stoppage time andthus ensures not only the production of a uniform thickness of zinc overthe entire substrate but also the quality of the deposition since thelatter is effected only in the presence of a plasma. It is well knownthat the presence of ions during a process of vacuum condensation of ametal on a substrate improves the quality of the coating.

This device for regulating the zinc vapour flow allows the regulation,to the two faces of the sheet, of a nominal flow of zinc vapour ofaround 3.5 kg per minute.

It goes without saying that the invention is not limited solely to thegeometries and mechanisms of the vapour flow regulation devicesdescribed above but also considers generally any geometries andmechanisms; for example, the obturation elements 13 could have theirrotation axis fixed in generally any direction and not only parallel tothe axis of travel of the substrate. Means other than the slats 15 orslides 18-21 can be used, for example rotary or iris diaphragms actuatedin the plane of the wall where they are provided, etc.

The invention claimed is:
 1. A method for the coating of runningsubstrates wherein the substrates are moved in a direction of travelthrough a treatment zone extending between an entry opening and an exitopening in a tubular passage provided in a confinement enclosure, thetubular passage being delimited by walls presenting a treatment opening,in which vapour of a coating material is generated in the confinementenclosure, wherein the vapour of the coating material present in theconfinement enclosure is freely circulated and is distributed aroundsaid tubular passage in a free space between walls of the confinementenclosure and the walls of the tubular passage comprising the treatmentzone, the vapour passing through the treatment opening towards thetreatment zone where the coating material condenses on the surface ofthe substrates, the passing of the vapour through the treatment openingtowards an inside of the tubular passage being controlled by regulatingobturation of the treatment opening by at least one obturation elementthat is connected to be movable between an open position, in which flowof said vapour from the confinement enclosure passes through thetreatment opening towards the tubular passage and the treatment zone,and a closed position in which the passing of the vapour towards thepassage through the treatment opening is prevented so that the treatmentzone is isolated with respect to the confinement enclosure and thevapour source.
 2. The method according to claim 1, in which thedistribution of the vapour through the treatment opening towards thetreatment zone is regulated according to the shape and/or dimensions ofthe cross section of the substrates to be coated.
 3. The methodaccording to claim 1, in which the obturation element is maintained at asufficiently high temperature to prevent condensation of the vapour ofthe coating material on this element.
 4. The method according to claim1, in which the walls of the confinement enclosure are heated to preventthe coating material being deposited on the walls.
 5. The methodaccording to claim 1, in which the confinement enclosure and thetreatment zone are maintained under vacuum, in particular at an argonpressure of less than 0.01 mbar.
 6. The method according to claim 1, inwhich the coating material is evaporated by a plasma produced in theconfinement enclosure and in which the coating material in the vapour ispolarised on average negatively with respect to a counter-electrode. 7.The method according to claim 1, in which the vapour of the coatingmaterial is formed from a metal maintained in the liquid state in theconfinement enclosure.
 8. The method according to claim 7, in which themetal is introduced into the confinement enclosure in a retention vesselforming a vapour source, from a metal reserve maintained in a vacuumfurnace.
 9. The method according to claim 7, wherein said metal is zinc.10. The method according to claim 8, wherein said metal is zinc.